1. Field of the Invention
This invention relates to an economical steel for being used to manufacture a reinforcement tube and the reinforcement tube manufactured from the steel.
2. Background Information
Steel reinforcement elements are typically installed to provide added strength to the door and provide passenger protection against lateral collision accidents involving motor vehicles. Various types of reinforcement elements have been used.
One type of steel reinforcement element is described in U.S. Pat. No. 4,090,734 to Inami et al. The reinforcement element of Inami et al. is in the form of a central piece formed from a plate of high tensile strength steel and two end pieces of a mild steel that is the same steel as the steel of which the door inner panel is made. The pieces of the reinforcement element of Inami et al. are given added strength by cross-sectional undulations which run the length of the pieces. The central reinforcement element is welded to the end pieces, which are then welded into a vehicle door.
One alternative type of reinforcement element is a tubular steel reinforcement element. The steel tubes used for the manufacture of tubular reinforcement elements must satisfy certain minimum requirements regarding strength, toughness and workability if they are to perform their task of absorbing as much of the impact energy as possible. For example, the following values, as a function of the size of the profile, must be achieved under specified test conditions:
______________________________________ Tensile strength (R.sub.m) at least - 1100 N/mm.sup.2 Yield strength (R.sub.t) at least - 800 N/mm.sup.2 Elongation at fracture (A.sub.5) at least - 8% Work of deformation (W.sub.150) at least - 1900 Joule ______________________________________
Such reinforcement tubes are conventionally manufactured by hot deformation, and the required strength of is achieved by water hardening. A steel of the prior art, for example, may contain 0.18% carbon, 0.4% silicon and 1.14% manganese, with the remainder being iron and usual impurities.
When the reinforcement tubes are processed for use in a vehicle, they are frequently subjected to treatments and processes which can have a significantly adverse effect on the original characteristics of the metal, such as those listed above. Examples of such processes are welding processes which, in conventional steels, generally lead to negative changes in characteristics in the heat-affected zone, and the subsequent galvanizing of a door structure for purposes of corrosion protection. In particular, hot dip galvanizing can be conducted at temperatures of up to approximately 500 degrees C. Subjecting the tubes to such temperatures represents a tempering treatment, which is in itself undesirable. For the reinforcement tubes which are in the hardened state, the tempering caused by the hot dip galvanizing is connected with the disadvantage of a corresponding loss of strength.
DE 37 28 476 Cl discloses the use of a steel alloy as a material for the manufacture of reinforcement tubes. The steel alloy disclosed has the following composition: (in weight percent, wt %)
______________________________________ max. 0.35% Carbon max. 0.50% Silicon max. 1.8% Manganese max. 0.030% Phosphorous max. 0.030% Sulfur 0-1.5% Nickel 1.8-2.2% Chromium 0.4-0.7% Molybdenum 0.025-0.050% Aluminum ______________________________________
Remainder iron and usual impurities.
The manufacture of the tubes by hot rolling is thereby carried out with the proviso that the final rolling temperature must be between 980 and 1080 degrees C. Since the steel is air-hardened, the above-mentioned minimum values of the manufactured tubes can be satisfied without the additional requirements of a complex and expensive heat treatment process. Nothing in DE 37 28 476 Cl discloses anything about a change of the material properties caused by hot dip galvanizing. One disadvantage of the alloy described therein is that expensive alloy elements such as Chromium, Nickel and Molybdenum must be used in rather large quantities, and thus, the increasing material costs, at least to some extent, offset the savings represented by the elimination of the heat treatment.